The present invention is directed to an improved electrochemical oxidation process for forming quinones and aromatic aldehydes or ketones from corresponding aromatic and alkyl aromatic compounds in good yields and high selectivity. More specifically, the invention described and claimed herein requires the use of an aqueous-organic cosolvent system having low concentrations of methanesulfonic acid therein and high concentrations of ceric methanesulfonate dispersed therein.
The quinones obtainable by the present process have a wide variety of known utility. For example, quinones, such as naphthoquinone, are known intermediates to dyes and are additives in the paper making industry. The aldehydes, such as benzaldehyde, tolualdehyde and the like, and ketones, such as p-methylacetophenone, are known intermediates used in forming fragrance components useful in perfumes and colognes. Certain aldehydes and ketones have been used in forming pharmaceuticals.
The products achieved by the present invention have been previously formed by a variety of processes which may be generally classified as chemical or electrochemical. For example, aromatic aldehydes have been chemically formed by air oxidation conducted in an oxygen enriched environment at high temperatures and pressure in the presence of a transition metal catalyst or by using known chemical oxidizing agents which are not regenerable. Oxidation has also been achieved by direct electrochemical oxidation of aromatic compounds in the presence of dilute acid electrolytic solutions as described in U.S. Pat. Nos. 4,298,438 and 4,354,904 and by indirect electrochemical oxidation in which the oxidant is electrolytically generated and, in turn, used to oxidize the aromatic compound.
Compounds which are known to be capable of acting as an indirect oxidant include transition metal salts, particularly the metals of cobalt, chromium, manganese, iron, lead, silver and cerium. Because regeneration of the spent metal to its higher oxidation state is not always highly effective and/or other insoluble salts, such as oxides, etc., are formed, those skilled in this art tend to use the salts of chromium, manganese, cobalt, iron or lead as these salts are less expensive and replacement of spent materials does not greatly detract from the economics of the process. However, each of these metal ion oxidants have certain properties which cause them to make the oxidation process ineffective. For example, chromium ions give poor selectivity towards the desired products, cerium and manganese salts are believed to have low solubility of the oxidized and/or reduced ions in acidic solutions causing reduced activity or separation problems, the higher oxidation states of silver, cobalt and lead ions are not very stable and, in the case of iron, is not very reactive. Indirect electrochemical oxidation has been further complicated by the properties of the anion specie present. For example, certain anions (e.g., chloride, nitrate, perchlorate) are highly reactive with the organic substrate producing by-products or conditions which preclude their use on a commercial scale. Other less reactive anions (e.g., sulfate, acetate, fluoride, boron fluoride, silicon fluoride) generally form salts of low solubility, inhibit the rate of reaction of the oxidant with the organic substrate and/or inhibit the ability of the spent oxidant to be regenerated. In addition, certain organic acid salts (e.g., benzenesulfonate) have been found to be insufficiently stable to be useful in an indirect oxidation process.
Ceric ion is a well known oxidizing agent in organic chemistry. It has the potential of presenting an excellent one electron oxidant but has not been previously used extensively or on an industrial scale because of the inability of both the ceric and cerous ions to be maintained in solution at high concentrations causing its use to be limited to slurries or very low concentrations with concomitant slow reactivity. The cerium salts are prohibitively expensive and, therefore, must be capable of being stable, achieving good reaction rates, reacting with the organic substrate cleanly and easily regenerating to its higher valence state. In addition, the cerous ion must be highly soluble to be capable of being regenerated to the ceric ion under conditions of high current efficiency in the anodic portion of the electrochemical cell. However, conditions preferred for best utilization of the ceric ion have previously been believed as being counterproductive to achieving proper conditions for cerous salt utilization. Therefore, it has heretofore been believed necessary to use the cerium salt at very low concentrations and under a very narrow set of conditions including those which could not demonstrate the potential necessary to provide an effective industrially suitable process.
Canadian Pat. No. 1,132.996, to Oehr describes a process for oxidizing naphthalene to naphthaquinone using ceric sulfate in dilute sulfuric acid. Both cerous sulfate and ceric sulfate are known to have low solubility in dilute acid [Solubilities of Inorganic and Organic Compounds, Vol. 3, Part I, Ed. by H. L. Silcock (1974)] and the solubility decreases with increasing acid concentration. The solubility limitations lead to the use of inefficient slurry conditions or to the need for large volumes of solution to oxidize small quantities of the organic compound. Similar problems are encountered with other salts of low solubility.
European Patent Application No. 0075828 of Mayeda et al describes a process for oxidizing fused ring compounds to their respective quinones using ceric nitrate in dilute nitric acid. Although solubility does not present a problem, the nitrate anion is known to react with the organic reactant forming nitrogen containing by-products which are difficult to handle and remove. Cerium salt solutions containing perchlorate anions have also been disclosed as a useful oxidant [Prospects for the Indirect Electrolytic Oxidation of Organics, by N. Ibl et al., AIChE Symposium Series, Electroorganic Synthesis Technology, Pg. 45, (1979)] but it is well known that the perchlorate reacts explosively with organic materials and, therefore, is unsuitable for commercial scale processes.
M. Marrocco et al [J. Org. Chem., Vol. 48, No. 9, Pg. 1487 (1983)] conducted a study of the oxidation of an organic substrate by various cerium salts in different acid electrolytes. Each of the cerium salt systems contained excess perchlorate or trifluoroacetate anions and the cerium ions were maintained at very low concentrations. Even at the low concentrations the systems were, in some instances, slurries. Of the systems examined, the cerium salt of trifluoroacetate in trifluoroacetic acid proved most effective although conversion and selectivity were still low. Several systems, including cerium perchlorate or trifluoroacetate in methanesulfonic acid, were shown to be ineffective.
The oxidation of naphthalene to naphthaquinone using an aqueous solution containing acetonitrile was reported by M. Periasamy et al, in Synthesis, Pg. 330 (1977). The process required the use of ceric ammonium sulfate in 4 normal sulfuric acid and acetonitrile. Although high yields were achieved, the system exhibited very low rates of reaction. The process required extended reaction time (6 hours) and isolation of the product was difficult due to the precipitation of the cerous salt out of solution as the oxidation proceeded.
The oxidation of aromatic and alkyl substituted aromatic compounds with ceric methanesulfonate is disclosed in applicant's copending application, U.S. Ser. No. 859,548 filed May 5, 1986, now U.S. Pat. No. 4,639,298. The process requires the use of an aqueous solution which maintains a high concentration of free methanesulfonic acid. This high acidity requirement has been found to inhibit formation of certain desired products.
It must be understood that although cerous/ceric ions have been known and used in oxidation reactions, there is a need to have a system wherein the ceric oxidant can be sufficiently stable under oxidizing conditions to be useful in indirect electrochemical processes, to be capable of undergoing repeated cycling between its cerous (Ce.sup.+3) and ceric (Ce.sup.+4) species in a high degree of efficiency under the reaction and electrolysis conditions, to be highly selective in forming the desired carbonyl group containing compounds, to be capable of exhibiting high reaction rates to make the process attractive on a commercial scale, to have high solubility of the cerous specie to aid in the efficiency of ceric regeneration and to provide an easy means for separation and recovery of the organic product. It is readily seen that a means of achieving this combination of desired properties would aid in providing a process which would find a high degree of acceptance in electrochemical oxidation of aromatic and alkyl substituted aromatic compounds.